Simple Summary: Over the past decade, camera traps have proven to be a valuable and efficient tool to collect data on cryptic and endangered animal species. Since its description in 2012, terrestrial camera trap surveys have been conducted on Cercopithecus lomamiensis (common name lesula), an endemic primate species of the central Congo Basin in the Democratic Republic of the Congo. The objective of this study was to use camera trap data to expand knowledge on the behavioral ecology of this cryptic species. We established two systematic camera trap grids inside the Lomami National Park and one outside the park in the buffer zone, where animals are heavily hunted. We confirmed that lesula's locomotor behavior is highly terrestrial, has a diurnal activity pattern, and lives in medium–large groups with one adult male, multiple adult females and their juveniles. Observations of young infants suggest peaks in births during the wet season. Our study provides new information about the behavioral ecology of this little-studied primate species, generating species-specific knowledge of a threatened species for successful conservation planning. Guenons are the most diverse clade of African primates, and many species living within the core of the Congo Basin rainforest are still understudied. The recently described guenon species, Cercopithecus lomamiensis, known as lesula, is a cryptic, semi-terrestrial species endemic to the central Congo Basin in the Democratic Republic of the Congo. The recent IUCN Red List Assessment recognizes lesula's risk of extinction in the wild as Vulnerable. The objective of our study was to use camera traps to expand knowledge on the behavioral ecology of lesula. We conducted three systematic, terrestrial camera trap (CT) surveys within Lomami National Park and buffer zone (Okulu: 2013; Losekola: 2014; E15: 2015). We accumulated 598 independent events of lesula over 5960 CT days from 92 CTs. Typical of Cercopithecus species, camera trap videos reveal that lesula has a diurnal activity pattern, birth seasonality, a group size of up to 32 individuals, and social organization with female philopatry and male dispersal. Results also suggest that lesula are highly terrestrial, distinguishing them from other Cercopithecus species, which are mostly arboreal. Our study provides new information about the behavioral ecology of this little-studied primate, generating species-specific knowledge of a threatened species for successful conservation planning. [ABSTRACT FROM AUTHOR]